Floor construction in building projects can involve the use of steel joists placed in different positions spanning structural supports and a concrete slab subsequently poured on decking supported by the joists. The slab and the joist form a composite structure having superior strength properties compared to a conventional non-composite floor system. Generally, the joist is in the form of a truss having top and bottom chords that are connected by a web. The web itself can vary in design by either being constructed of solid material or comprising tension and compression members triangulating the space between the top and bottom chords.
U.S. Pat. No. 4,729,201 granted to Laurus et al. discloses a joist consisting of a web, a bottom chord and a double top chord construction consisting of two elongated substantially identical portions each being of S cross-section, extending the length of the joist. This design of the top chord improves the lateral stiffness which improves the strength of the composite structure during construction stages and permits safer construction procedures particularly where long spans are involved. The symmetry of the top chord provides structural advantages during the installation stage where the unpropped joist is required to carry the weight of wet concrete, form work, its own weight and other construction live loads. The serpentine top chord profile provides a cross-sectional area whereby the resulting lateral slenderness properties of the joist are improved and the joist is consequently stiffened. The increased stiffness improves the resistance of the joist to compressive stresses.
Several other prior art documents describe various types of joist designs for use in composite floor systems. These documents are known to the Applicant by the following patent numbers: U.S. Pat. No. 5,544,464 (Dutil), U.S. Pat. No. 6,061,992 (Vincent), US 2003/0024205 A1 (Strickland), CA U.S. Pat. No. 2,404,535 (Moreau), US 2003/0084629 (Strickland et al.), U.S. Pat. No. 4,454,695 (Person), U.S. Pat. No. 4,700,519 (Person et al.) and U.S. Pat. No. 4,592,184 (Person et al.).
A symmetrical shape of the joist also helps avoid such problems as sweep. Sweep is a phenomenon that is encountered during construction of a welded joint between the chord and the web of the joist, where all welds are made on one side of the web. This welding can create a stress in the joint which tends to result in a certain amount of curvature in the completed joist. This curvature effect can usually be avoided. This was previously accomplished by pre-curving of the top chord in a direction opposite of that of the sweep effect. The pre-curving of the top chord results in a substantially straight longitudinal configuration for the completed joist once welded together. A symmetrical top chord avoids this entire problem of sweep as a straight joint is obtained without the necessity for complicated pre-assembly compensation techniques. Symmetry in the top chord also doubles the amount of shear bond between the joist and the concrete slab.
However, during the fabrication process for steel joists, access to the area to be welded between the chord and the web element is difficult due to the overall shape of the serpentine top chords that are currently used in different joist designs. The shape of the top chord in these designs prevents a welding tool from being inserted between the two leg elements of the chord from a point directly above the joist. This problem complicates the joist welding process during manufacturing of joists in an assembly line.
Thus, there is still presently a need for a top chord design that would facilitate this welding process during fabrication of joists without losing the advantages of symmetrical top chords.